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The Chinchorro people lived on the coast of the Atacama Desert, in modern-day Peru and Chile.

Archaeologists have long wondered what spurred these hunter-gatherers to start preserving their dead. A new study points to a changing climate.

The oldest Chinchorro mummies date back about 7,000 years, and they show a surprising level of sophistication.

Bernardo Arriaza, a physical anthropologist at Chile’s University of Tarapaca and an author of the new study, says the early Chinchorro people followed an elaborate process to mummify the dead.

“They removed the organs, they cleaned the cavities,” Arriaza said.

They packed the bodies with clay, sewed the skin back, and painted the bodies black from head to toe. Then they placed wigs on the heads, and they left the eyes and mouths open.

Unlike other societies that preserved their dead, Arriaza says, the Chinchorro didn’t mummify only their elite.

“Everybody was being mummified in similar ways,” he said. “Older people, younger people, even fetuses and newborns.”

But why did this simple society of hunter-gatherers and fishermen engage in this time-consuming, sophisticated practice?

There have been many theories, but none of them have explained why the practice began when it did. Arriaza and his colleagues suspected there might have been a change in the environment at that time.

So for their new study, they looked at clues to the region’s ancient climate collected by other scientists.

They found that a couple of centuries before the Chinchorro started mummifying their dead, the climate became less harsh in the region. That meant more fresh water and food for the people. The result was a growing population of people.

“A higher population density of living people also means that there were more dead people,” said ecologist Pablo Marquet of Ponitifica Católica de Chile (Chilean Catholic University).

In any other place, having more dead people might not mean much. But this was the Atacama desert, one of the driest places on the planet.

“Once you die, you naturally mummify,” Marquet said. “Because it’s very dry, corpses do not decompose. So you stick around.”

What’s more, the Chinchorro didn’t bury their dead very deep, and the bodies could have easily been exposed by wind.

Marquet and his colleagues theorize the Chinchorro lived in a landscape littered with naturally preserved dead bodies.

“Dead people became a very significant part of the physical landscape of the living, and also the psychological landscape of the living people,” he said.

Of course, it’s impossible to know what the Chinchorro believed, but Marquet and his colleagues suspect that the Chinchorro — seeing their dead ancestors and their deceased loved ones on a daily basis — began to regard the dead as another dimension of the living.

That might have led them to put more care into preparing and decorating their dead, and preserving them as mummies.

Christina Warinner, an archaeologist at the University of Zurich, finds that part of the new theory plausible. What she finds more convincing and exciting is the link to the change in the climate.

“Traditionally we think of only agricultural societies as societies that develop complex burial and mortuary practices,” she said. “But the real secret is not really agriculture, but having a stable food base.”

Bolstering that argument is what happened when the climate changed again.

About four thousand years ago, the climate became drier, and food became more scarce. And it’s around that time that the Chinchorro stopped mummifying their dead.